1. Field of the Invention
The present invention relates to a, radio data communications method, a server, and a radio network controller.
2. Description of the Related Art
The Universal Mobile Telecommunication System (UMTS) has been known as a radio communications system standardized by the 3rd Generation Partnership Project (3GPP).
The UMTS adopts W-CDMA as a radio communication technology, and provides soft handover (diversity handover) as one of handover methods of mobile terminals. The soft handover has an advantage that a mobile terminal can be simultaneously connected to a plurality of base stations for communication, performing a handover without causing loss of data.
With reference to FIGS. 1 to 3, a soft handover process for allowing the soft handover of the mobile terminals in the UMTS will be described.
As shown in FIG. 1, a UMTS network consists of a core network including Mobile services Switching Centers (MSCs)/Serving GPRS Support Nodes (SGSNs) and a Gateway MSC (GMSC)/Gateway GPRS Support Node (GGSN), and a Radio Access Network (RAN) including Radio Network Controllers (RNCs) and base station Node Bs.
In the UMTS network, the soft handover process is performed in the RAN.
As shown in FIG. 1, an RNC 1 located in a data transmission/reception path (route) for radio communication started by a mobile terminal MN (Mobile Node) 1 becomes a Serving-RNC (SRNC) for the radio communication of the mobile terminal MN 1, for example. The SRNC performs the soft handover process for the mobile terminal MN 1. Here, there is only a single SRNC for given radio communication.
The soft handover process required for the implementation of soft handover in downlink radio data communications includes as follows:    the measurement of data delays between Node Bs (connecting Nodes B2, B3) to which the mobile terminal MN 1 is connected and the SRNC, that is to say, the process required for the mobile terminal MN 1 to receive data from the plurality of Node B2, B3 simultaneously, or for the arrival synchronization control;    the measurement of timing differences between a clock held by the SRNC and clocks held by the connecting Nodes B2, B3;    the determination and instruction of the timings of transmission from the SRNC to the connecting Nodes B2, B3;    the determination and instruction of the timings of transmission from the connecting Nodes B2, B3 to the mobile terminal MN 1;    the instruction of reception timings from the connecting Nodes B2, B3 to the mobile terminal MN 1;    the division of data in L3 frame format received from the MSC/SGSN into data fragments in L2 frame format;    the provision of sequence numbers required for associating the data fragments (in L2 frame format) with the transmission timings;    the duplication of the data fragments a number of times equal to the number of the connecting Nodes B2, B3; and    the transmission of the data fragments based on the transmission timings.
The soft handover process required for the implementation of soft handover in uplink radio data communications includes as follows:    the selective combination of data (in L2 frame format) transmitted from the mobile terminal MN 1 via the connecting Nodes B2, B3;    the control of retransmission in L2 frame unit between the mobile terminal MN 1 and the SRNC when necessary; and    the reconstruction control for assembling L2 frame format data fragments after the selective combination (or retransmission control) into L3 frame format data.
With reference to FIGS. 2 and 3, the soft handover process for the mobile terminal MN 1 performed by the RNC 1 as an SRNC in the UMTS network shown in FIG. 1 will be described with an example in which the mobile terminal MN 1 initiates radio data communication and the mobile terminal MN 1 initiates soft handover to the Node B3 (or the mobile terminal MN 1 adds a branch to the Node B3).
First, with reference to FIG. 2, the process in downlink radio data communications will be described.
In step 1001, at the start of radio data communication, the SRNC (i.e., the RNC 1) measures a data delay between the SRNC and the Node B2, and a timing difference between a clock held by the SRNC and a clock held by the Node B2. The measurement may have been performed during the building of the system (hereinafter, the same is true).
In step 1002, the SRNC determines the timing of transmission from the SRNC to the Node B2 (at what value of the clock of the SRNC, what sequence number of data is to be transmitted), the timing of transmission from the Node B2 to the mobile terminal MN 1 (at what value of the clock of the Node B2, what sequence number of data is to be transmitted), and the timing of reception by the mobile terminal MN 1 (at what value of the clock given by the Node B2, what sequence number of data is to be received).
In step 1003, the SRNC notifies the mobile terminal MN 1 of the timing of reception by the mobile terminal MN 1. In step 1004, the SRNC notifies the Node B2 of the timing of transmission from the Node B2 to the mobile terminal MN 1.
In step 1005, the SRNC receives data in L3 frame format from the MSC/SGSN 1, and in step 1006, the SRNC divides the L3 frame format data into L2 frame format data fragments, and provides a sequence number to each data fragment.
In step 1007, the SRNC transmits the data fragments (in L2 frame format) to the Node B2 at the timing of transmission from the SRNC to the Node B2 determined in step 1002. In step 1008, the Node B2 transmits the data fragments (in L2 frame format) to the mobile terminal MN 1 at the timing of transmission from the Node B2 to the mobile terminal MN 1 given in step 1004.
Then, in step 1011, when adding a branch to the Node B3, the mobile terminal MN 1 monitors the radio environment between the mobile terminal MN 1 and the Node B3, and detects that the radio environment between the mobile terminal MN 1 and the Node B3 becomes better. In step 1012, the mobile terminal MN 1 reports the fact to the SRNC.
In step 1013, the mobile terminal MN 1 measures a timing difference between the clock given by the Node B2 and the clock given by the Node B3, and notifies the SRNC of it.
In step 1014, the SRNC measures a data delay between the SRNC and the Node B3 and a timing difference between the clock held by the SRNC and the clock held by the Node B3.
In step 1015, based on the measurement, the SRNC determines the timing of transmission from the Node B3 to the mobile terminal MN 1 and the timing of transmission from the SRNC to the Node B3 such that the mobile terminal MN 1 can receive the same data from the Node B2 and the Node B3 at the same timing.
In step 1016, the SRNC notifies the Node B3 of the timing of transmission from the Node B3 to the mobile terminal MN 1.
In step 1017, the SRNC receives data in L3 frame format from the MSC/SGSN 1, and in step 1018, the SRNC divides the L3 frame format data into L2 frame format data fragments, provides sequence numbers to the data fragments based on a sequence number providing status, and generates two sets of the data fragments by duplication for transmitting the data fragments to the Node B2 and the Node B3.
In step 1019, the SRNC transmits the two sets of data fragments (in L2 frame format) to the Node B2 and the Node B3 at the above transmission timings, respectively. In step 1020, the Node B2 and the Node B3 transmit the data fragments to the mobile terminal MN 1 at the above transmission timings, respectively.
As a result, the mobile terminal MN 1 can receive the same data from the Node B2 and the Node B3 simultaneously.
Second, with reference to FIG. 3, the process in uplink radio data communications will be described.
In steps 1101a and 1101b, at the start of radio data communication, data in L2 frame format transmitted from the mobile terminal MN 1 is transmitted only through the Node B2 to the SRNC. Here the mobile terminal MN 1 divides L3 frame format data into L2 frame format data fragments, and provides a sequence number to each data fragment for transmission.
In step 1102, the SRNC performs retransmission control on the data received via the Node B2, between the SRNC and the mobile terminal MN 1 when necessary.
In step 1103, the SRNC assembles the L2 frame format data fragments so as to reconstruct original L3 frame format data, and in step 1104, the SRNC transmits the reconstructed L3 frame format data to the MSC/SGSN 1.
Then, in steps 1111 and 1112, when the mobile terminal MN 1 adds a branch to the Node B3, L2 frame format data from the mobile terminal MN 1 is transmitted to the SRNC via the Node B2 and the Node B3.
In step 1113, the SRNC performs a selective combination of the received L2 frame format data (data fragments) having the same sequence numbers, and if necessary, performs retransmission control between the SRNC and the mobile terminal MN 1, and assembles the selectively combined L2 frame format data fragments so as to reconstruct original L3 frame format data.
In step 1114, the SRNC transmits the reconstructed L3 frame data to the MSC/SGSN 1.
As a result, data from the Node B2 and the Node B3 can be put together for transmission to a corresponding node CN 1.
As described above, in the conventional UMTS, the soft handover process is fixedly performed at a single SRNC, and the SRNC performing the soft handover process is not changed during the radio data communication.
When the mobile terminal MN 1 performs a handover across RNCs, a subscriber's line extension system is adopted, and data transmission and reception to and from Node Bs is always performed via an SRNC.
In FIG. 1, downlink data and uplink data between the RNC 1 as an SRNC and the Node B3 is transmitted and received via the MSC/SGSN 1 and the RNC 2, for example. The RNC 2, however, only relays the data, and the soft handover process is still performed only by the RNC 1 as an SRNC.
The above-described conventional art, however, has a problem in that it does not specify a method of taking over control for relocating a control point (SRNC) during communication under soft handover. This is because, in the UMTS, it is determined that only one of RNCs in a network having a hierarchical configuration performs a soft handover process, and the RNC performing the soft handover process is not changed during communication.
In the UMTS, an “SRNC Relocation” method is specified as a method of switching data transmission and reception paths during communication.
The “SRNC Relocation” method, however, is not for soft handover, and has a problem of possibly causing loss of data during switching of data transmission and reception paths.
Suppose, for example, that it is possible to construct a flat network (router network) in which there is no distinction between exchanges and RNCs for a mobile communications network as an IP network, and to perform a soft handover process at any control point in the network.
When, for example, there occurs an alternating path including a redundant part like a path “A” shown in FIG. 1 in the subscriber's line extension method, it is very effective in terms of effective use of network resources to switch a point (control point) for switching a data transmission and reception path to a location corresponding to the MSC/SGSN 1, so as to optimize the path. However, as described above, in the UMTS, it is impossible to optimize the path like that.